Typically, energy is not in readily utilizable forms. Many means exist for converting one type of energy to another. For example, an internal combustion engine can turn the explosive force of a fuel burned in its cylinders into mechanical energy that eventually turns the wheels of a vehicle to propel a vehicle. An internal combustion engine channels energy resulting from the burning of a fuel in a cylinder into a piston. Without the cylinder and piston, the energy resulting from the burning of the gas would simply spread out in every available direction. Another example of a device to convert one form of energy into another is a windmill. If connected to an electric generator, windmills can convert the mechanical action of moving air into electricity.
While an internal combustion engine typically produces mechanical energy from the burning of fossil fuels, such as gasoline, diesel fuel, or natural gas or alcohols, other attempts have been made to produce mechanical energy from the movement of members such as pistons by means other than the burning of fossils fuels. However, most of these devices still operate on the basic principle of providing a force to drive a moveable member such as a piston. The difference among the various devices in the way in which the force is produced to move the piston and the way in which the force is controlled.
Some of these devices utilize the movement of a working fluid to drive a moveable member, such as a piston. Other devices utilize the phase change in a liquid to drive a moveable member. In their operation, some devices utilize valves to control the flow of a working fluid in the production of mechanical energy by moving a moveable member.
Due to the worldwide and ever increasing demand, research constantly focuses on ways to produce energy or power the devices that we rely on in our daily lives. In recent years, another area of research has included alternative sources of energy. Such research has constantly increased. Among the reasons for the increased research is an increased awareness of the limited amount of fossil fuels in the earth. This research may also be spawned by an increased desire to provide energy for people living in remote locations around the world who now live without power.
Among the alternative sources of energy on which research has been focused is solar energy. Solar energy has been captured by photovoltaic cells that convert the sun's energy directly into electricity. Solar energy research is also focused on devices that capture the sun's heat for use in a variety of ways.
As discussed above, in relation to the internal combustion engines and windmill examples, the problem being addressed both by photovoltaic solar cells and solar heating devices is the conversion of one type of energy to another type of energy. In solar cells, the energy in sunlight is used to excite electrons in the solar cells, thereby converting the sun's energy to electrical energy. On the other hand, in solar heating cells, the energy of the sun is typically captured by a fluid, such as solar hot water panels typically seen on the rooftops of residences.